This invention relates to a process for treatment and separation of a mixture comprising O,O-di(C.sub.1 -C.sub.8 -alkyl) phosphorochloridothioates and sulfur. The O,O-dialkyl phosphorochloridothioates are valuable intermediates, for instance, in the preparation of insecticides. For instance, O,O-diethyl thiophosphoryl chloride is an intermediate in the synthesis of insecticide known as parathion, and O,O-dimethyl thiophosphoryl chloride is an intermediate in the synthesis of the insecticide called methyl parathion.
Several methods have been used for the synthesis of the esters of phosphorochloridothioic acid including one-step and two-step methods. In the one-step process, phosphorus pentasulfide, alcohol and chlorine are reacted to prepare the ester corresponding to the alcohol and then the solvent is removed and the product distilled.
In the two-step process, the first process step reacts phosphorus pentasulfide with an alcohol, such as ethanol, so as to form O,O-diethyl dithiophosphoric acid and hydrogen sulfide, and in a second process step the isolated O,O-diethyl dithiophosphoric acid is chlorinated in an appropriate solvent with chlorine gas, resulting in the formation of O,O-diethyl thiophosphoric acid chloride. Conventionally, the alkyl groups in the dialkyl phosphorochloridothioates have from 1 to 8 carbon atoms and are generally selected from methyl, ethyl, isopropyl, butyl, sec.-butyl, t-butyl, and the like, up through n-octyl and isomers thereof. However, each of these conventional one-step and two-step processes produces sulfur in some form which must be separated from the product. Several solutions to the sulfur problem have been proposed in the prior art. In one process, the product is simply distilled under reduced pressure from the reaction vessel. The sump temperature increases to about 150.degree. during distillation, and upon cooling after terminating distillation the liquid sump phase solidifies and consists essentially of elementary sulfur (confer U.S. Pat. No. 3,356,774).
In another prior art patent disclosing a one-step process, the reaction mixture is treated with hydrogen sulfide to convert the sulfur monochloride formed during the reaction of chlorine with the dialkyl dithiophosphoric acid. When the hydrogen sulfide treatment is carried out at relatively low temperature, a precipitate of sulfur is obtained with practically no by-products. Then, by distilling under vacuum and washing the distillate with water, the diesters of phosphorochloridothioic acid are obtained in very high yield and very high degree of purity (U.S. Pat. No. 3,502,750). In another prior art process using a two-stage chlorination reaction technique, the reaction mixture is chlorinated and then established and maintained at a temperature in the range of 85.degree.-110.degree. C. until it is substantially free of sulfur monochloride and the relatively thermal unstable sulfur that forms becomes more thermally stable so that the product dialkyl thiophosphoryl chloride can be readily and safely removed from the mixture thereof with sulfur by distillation (U.S. Pat. No. 3,836,610). Also, the chlorination-heat treating process can be conducted in a two-stage operation employing first a high temperature to solubilize sulfur, followed by a low temperature to precipitate sulfur from solution. After the high temperature range is established and maintained so that sulfur monochloride is changed to a thermally stable condition, preferably to sulfur and the sulfur goes into solution without substantial decomposition of the phosphoridothioates, the resulting solution is then established at a temperature at which the dissolved sulfur crystallizes, precipitates from solution and is then separated from the solution by settlement, e.g., using filtration, decantation, centrifugation and the like (U.S. Pat. No. 3,856,898).
However, in all of these processes, whether by distillation or by solid/liquid separation, small amounts of free sulfur still remain with the product. Such small amounts affect product purity and have adverse effects on subsequent reactions or may not provide products which meet commercial specifications for production of commercial insecticides. Further, the handling of solid sulfur residues remaining in the reaction vessel is extremely difficult. Batch distillation processes are required because the amounts of sulfur remaining tend to solidify and plug continuous distillation columns. Finally, product losses from the batch vacuum and steam distillation processes are higher than one would like to incur.